The present invention relates to a braking-force booster containing a pressure source, a pedal-operated control valve and at least one main brake cylinder, the braking-force generating piston is subjected to the pressure generated in the main brake cylinder by means of the control valve, wherein the control valve in accordance with the pedal deflection closes a first connecting passage between the main brake cylinder and a pressureless connection and temporarily creates a second connecting passage between the pressure source and the main brake cylinder.
A braking-force booster with the above characteristics is known, for instance, from German Offenlegungsschrift No. 2 344 969, wherein a tandem main brake cylinder for dual brake circuits is provided. In this known apparatus the brake valve and the tandem main brake cylinder are shown as separate components. It is known to house the two components of essentially equal functional construction in one unit.